shear and pressure forces, electric fields, magnetic fields, acoustic fields, and acceleration. Other examples are spreading, stability, and rupture of ultrathin liquid films. Much can be learned by considering granular materials at reduced-gravity. Key aspects concern effects such as particle clustering, self-assembly, and dissipation; the study of electrostatic effects and interstitial fluids; and the impact of having multiple particle sizes and/or shapes.
In the area of materials synthesis and processing, a microgravity environment can shed new light on the nucleation process because liquids can be suspended and solidified without a container, thus removing the effects of walls, as well as convection due to compositional inhomogeneities that accompany the formation of nuclei. Thus it is possible to study the formation of stable and metastable phases from undercooled melts, the formation of glasses, the relationship between liquid structure and the resulting crystal structure, and the thermophysical properties of deeply undercooled liquids. Understanding the processes leading to the production of materials composed of phases with much different densities, such as metallic and ceramic foams, can be improved by research on the ISS.
On Earth, during crystal growth the density differences between crystals and the parent fluid or vapor—as well as the temperature and composition dependence of the density of the parent phase and variations in the surface tension of a liquid-vapor—lead to convection. This convection results in nonuniform compositions as well as defects in the resulting crystal. The microgravity environment allows these crystal growth phenomena to be studied without the confounding effects of gravitationally induced convection. The Materials Science Research Rack (MSRR) available on the ISS is a very valuable asset.
Gravitationally induced convection or sedimentation makes it very difficult to study the physics that underlie processes such as dendritic and cellular solidification, liquid phase sintering, and phase separation. The effects of interactions between individual dendrites or cells on their spatial distribution and morphology, the evolution of dendrite morphology during transient heating or cooling, and the effects of noise and initial conditions on the resulting patterns remain unclear. The interactions between dendrites are particularly important in setting the properties of a solid-liquid mixture found in castings, called the mushy zone. Fluid flow within mushy zones can become unstable during solidification, resulting in deleterious casting defects. The nature of this instability and the properties of the mushy zone need further investigation.
Studies of combustion in a reduced-gravity environment would lead to a greater understanding of terrestrial combustion. On Earth, energy release, fluid dynamics, and gravity-induced buoyancy interact in a nonlinear fashion. By varying or eliminating the effects of gravity, researchers can extract fundamental data that are important for understanding combustion systems. Such data include parameters such as chemical reaction rates, diffusion coefficients, and radiation coefficients that strongly influence ignition, propagation, and extinction of combustion waves.
It is very rare, on Earth and in space, for an area, cabin, or room to be uniformly filled with a stoichiometric, homogeneous mix of fuel and oxidizer. Unfortunately, very little is known about the behavior of flames propagating through reactivity gradients. Reactivity gradients are important for all stages of a fire or explosion from ignition and propagation through to extinction. Reduced-gravity environments can be used to learn more about flame ignition, propagation, and extinction in reactivity gradients.
It is now speculated that gaseous flammability limits might not exist at all, or that a diffusive or hydrodynamic mechanism may cause extinction, or that flame balls or flame strings are themselves the limiting structure. Most combustors and unwanted fires involve diffusion flames. There remain significant gaps in the understanding of these flames, such as those associated with chemical kinetics, transport, radiation, soot formation, pollutant emissions, flame stability, and extinction. All of these areas will benefit from experiments performed on the ISS.